WEARING SURFACE FOR TYRES FOR WINTER CONDITIONS

Abstract: A wearing surface for tyres, especially for use on icy roads, of the kind where the rubber or rubber mix is supplied with an admixture of friction improving materials. The friction improving materials constitute approximately 20-60 % by weight of the rubber mix, and consist of a mixture of crystalline chrysolite (olivine) powder and a metal powder, being for example a steel powder and/or a powder from an aluminium alloy, and if desired an additional friction supporting material having good binding force to the rubber mixture.

Description: Wearing surface for tyres for winter conditions The invention under consideration is a composite material for use as wearing surface on rubber tyres for winter condi- tions. There are several factors influencing the grip of a car tyre on ice and snow, such as the tyre design, the non skid pattern as well as the material composition for the wearing surface. During the past years a lot of research work and money have been spent on developing tyres with improved performance, especially for winter driving under icy and slippery road conditions. Especially due to the heavy wear of pavement material experienced with the use of cogged tyres and resul- ting environmental problems, efforts have been made to develop tyres for improved performance without using cogs. A lot of design changes in the non skid patterns have been tried, as well as different rubber mixtures for the wearing surface with or without socalled friction increasing elements like grained or particle formed materials embedded. Examples of the present position of developments may be found by studying the following patent publications: US 2582701, US 2672910, US 2675047, US 2766800, US 3062255, GB 1578077, EP 442155, FR 2317109, WO 85/05329, DE 2101224, DE 3220603, US 1578121 as well as NO applications 903245 and 903259. The applicant has performed a number of practical tests covering different rubber mixtures as well as variable propor¬ tions of rubber and particles in the wearing surface. Through this work, the applicant has reached the conclusion that parameters like: -range of particle hardness -particle size and distribution -percentage of particles mixed into the wearing surface material are essential for the friction of the tyre against icy and slippery road surfaces. Depending on the hardness level of the material to be mixed in, the applicant has found that the percentage of particles mixed into the wearing surface materi¬ al may be considerably increased by reducing the grain size. Generally speaking, the frictional material for mixing into a wearing surface rubber compound is according to the present invention characterized by a mixture of crystalline chrysolite (olivine) and a metal powder, being for example a steel powder and/or a powder from an aluminium alloy, and if desired an additional friction supporting material having good binding force to the rubber mixture. A proposed admixture for the frictional material is 20 to 60% by weight, preferably approximately 45% by weight of the total wearing surface compound. Regarding composition of the frictional material it is suggested, after performing experiments, that the best results are obtained using approximate values of powder materials in the order of 5 - 20% steel, 30 - 60% crystalline olivine and additionally, if desired, some powder material. Regarding particle size it is concluded, after performing experiments, that the materials be finely ground to the extent possible. The olivine powder as well as the other materials used should as an example maintain a particle size of 0.001 to 0.1 mm, the greater part of the particles being preferably in the area 0.0015 to 0.022 mm. By using a mixture as mentioned above, a particularly stabilized relation between rubber- and friction material will occur due to the nature of the compound and the grading as well. As to the admixture of additional material, this part may as an example consist of coke powder. This coke powder may also partly or as a whole be exchanged by a silicon carbide in powder form. The use of coke powder in the friction material offers particularly good frictional properties, but the coke powder by itself reduces the wear resistance of the rubber compound. The olivine powder has proved to give a wearing surface with very good frictional properties in addition to the fact that the particle shape is positive for the binding to rubber. It has further been found possible to achieve a very good and homogenous mixture of olivine and steel powder, and the steel has been seen during the vulcanisation process to increase the heat conducting properties of the mixture quite considerably, leading to a better and smoother vulcani¬ zation of the tread to the cord. The steel powder also assists in improving wear resistance and steadiness of the tread. The raw materials of the friction compound are initially mixed in dry condition in a drum mixer or similar for approxi¬ mately 10 minutes. During this dry mixing process, the oli¬ vine powder has demonstrated especially good qualities since it assists in removing possible oxides or other surface coat¬ ings from the steel particles. This is important because any kind of coating on the steel particles will reduce the final binding to the rubber mixture. Subsequently a socalled "pri¬ mer", being in itself of a known type and make, is added, and the stirring is continued for for instance 30 minutes until the primer, originally having a liquid consistence, has been completely mixed with the solid materials and has dried. The treatment with primer is preferably done in two steps with a drying period in between. Primers well suited for the purpose are marketed under the product names Che osil 211 and Chemosil 220. After the friction compound is readily coated with primer which has been allowed to dry completely, it is mixed into the ordinary wearing surface rubber of the tyre. A preferred way of producing wearing surfaces is accord¬ ing to this invention to feed the friction compound into a mixing machine where it is mixed with the rubber. The final mixture is delivered from the mixing machine in pieces or strips of dimensions suitable for feeding an ordinary extruder where the compound is further extruded to a strip suitable for forming the tread on the tyre. After a strip of thickness suitable to form the tread has been applied on the tyre, the tread joint is rubbed down using a rowel, and the tyre is then shifted to a pressurized and heated vulcanizing form in order to press the tread pattern and vulcanize the tread to the tyre in an ordinary and well known manner. Concerning the steel materials used, these may very well be stainless. Provided the cost of stainless steel is consid- ered being on the higher side, some other high grade steel may as well be considered used, examples may be steel powder qualities marketed under the product names FLEXGRAIN or FERRO- GENT. Steel powder may also be substituted partly or totally by a granulated or powdered light metal alloy like for in- stance aluminium alloy AlMg 5 (NS 17552) . The admixture of friction material into the rubber mix may be performed in different ways. At the rubber manufac¬ turer, the friction material may for instance be mixed into the rubber while the latter is still flowing. Alternatively the friction material may be rolled into the rubber mixture while the latter is more or less flowing. As mentioned earlier, friction material as much as 20 - 60% by weight of the finished rubber mixture into which it is going to be admixed, should be used, preferably the percentage should be around 45. An important feature of the invention is that the steel powder used is of a grain size similar to the remaining mate¬ rials confirming the friction compound. Preferably the pro¬ portions by weight of the mixture should be 1 part steel powder to 7 parts of the remaining friction compound. This reads a single steel particle may be surrounded by 7 other particles from the friction compound, leading to an improved vulcanization of the entire friction compound into the rubber mix since the heat conduction of steel is better than for the other materials. The rubber is thereby more or less melted onto the particles of the friction material during vulcaniza¬ tion. Experiments have shown that by using metal powder in the form of an aluminium alloy, the heat conduction from the tyre is improved. Less problems with the tyre running hot and also reduced wear are therefore expected as a result. Regarding tread patterns of current interest, the appli¬ cant has arrived at the conclusion that, when using the inven¬ ted compound for the wearing surface, the tread pattern may with advantage be designed with less number of grooves, diago- nally as well as radially, so leading to an increased contact area between tyre and road surface. This is due to the new compound increasing the adhesion against ice as compared to what has been the fact for tyres till now. Through experi¬ ments, the applicant has reached the conclusion that a speci- ally favourable tread pattern may consist of substantially square pieces of material having a flat contact surface against the road, and being dissociated by grooves at right angles to each other in such a way that a set of parallel grooves are running in the longitudinal direction (diagonal grooves) , whereas another set are running in the transversal direction of the tyre wearing surface (radial grooves) . The groove width should be waning towards the bottom of the groove, for instance 7 mm width at the top decreasing to approximately 5 mm at the bottom. By this, occurrences like slash, snow or ice choking the grooves may be avoided. A tread pattern preferred by the inventor comprises square blocks having the approximate size of 20 x 20 mm, separated by grooves of approximately 5 mm width for fast drainage of water. Car tyres having a wearing surface compound consisting of the present invention, may be manufactured as ordinary new tyres or by retreading of second hand tyres. The manufacturing of a car tyre by retreading using the present invented compound, is illustrated by the enclosed figures where: Fig. l displays the cross-section of a car tyre having a wearing surface applied in layers using a compound similar to the present invention, Fig. 2 shows a magnified sector from the Fig. 1 tyre, whereas Fig. 3 shows a sector from a tyre as described by the inventor, as seen towards the wearing surface from above. Fig. 1 illustrates an ordinary method used for retreading of tyres, coiling of the desired number of layers - in this case 5 - of an endless strip of wearing surface material 2. When manufacturing tyres according to the present invention, a wearing surface material may be used that has got the desired admixture of grained or powder material already in the strip production phase. As mentioned earlier, the new wearing surface is vulca¬ nized to the tyre in a vulcanizing form at a temperature of typically 150 - 165 °C and at a pressure of 7 - 25 bars. Samples cut from tyres retreaded by applying multiple layers of thin strip show, according to the inventor, that the fric¬ tion compound has been distributed to neighbouring layers of the wearing surface in such a way that the entire wearing surface has got an almost homogenous material structure, schematically illustrated also in Fig. 2. It is appropriate, therefore, to squeeze out the tread pattern in the usual and well known manner during vulcanization without destroying the compound in any way. As mentioned before, experiments have shown that tyres containing the invented compound in the wearing surface may as well have a relatively bigger pattern contact area. A prefer¬ red tread pattern design as proposed by the inventor, is a checked one having the square form 6 shown in Fig. 3 with longitudinal and transversal intermediate grooves for the purpose of good water drainage. Strip- and belt formed wear¬ ing surface rubber for use during retreading of tyres are usually supplied in different thicknesses ranging from 2 to 3- 4 mm. A common thickness range is 3-4 mm. A good mix of the wearing surface compound has been obtained when applied in layers down to a thickness of 2-4 mm. Having built up a sufficient thickness of the wearing surface using strips of belt, the tyre wearing surface is smoothened and the tyre is shifted to a vulcanizing form for vulcanization.

Claims: PATENT CLAIMS 1. Wearing surface for tyres, especially for use on icy roads, of the kind where the rubber or rubber mix is supplied with an admixture of friction improving materials, c h a r a c t e r i z e d by the friction improving materials constituting approximately 20-60% by weight of the rubber mix, and consisting of a mixture of crystalline chrysolite (oli¬ vine) powder and a metal powder, being for example a steel powder and/or a powder from an aluminium alloy, and if desired an additional friction supporting material having good binding force to the rubber mixture. 2. Wearing surface for tyres as stated in claim 1, c h a r a c t e r i z e d by the friction improving materials constituting a friction compound consisting of approximately 5-20% powdered or granulated steel material, approximately 30- 60% crystalline chrysolite powder, and if desired some other material. 3. Wearing surface for tyres as stated in- claim 1, c h a r a c t e r i z e d by the metal powder in its entirety consisting of a powder from an aluminium alloy. 4. Wearing surface for tyres as stated in any of claims 1-3, c h a r a c t e r i z e d by the additional material consis¬ ting of coke powder. 5. Wearing surface for tyres as stated in claims 1, 2 or 4, c h a r a c t e r i z e d by the steel material consisting of stainless steel. 6. Wearing surface for tyres as stated in any of claims 1-5, c h a r a c t e r i z e d by the friction compound also being supplied with some part of silicon carbide, if desired in replacement of for instance 3-40% of the coke powder otherwise used. 7. Wearing surface for tyres as stated in any of claims 1-6, c h a r a c t e r i z e d by the chrysolite powder and pre¬ ferably also the additional materials used in the friction compound, having a particle size in the range 0.001-0.1 mm, while the major portion of the materials preferably maintains a particle size in the range of 0.0015-0.022 mm.

Inventors: CHRISTENSEN ERLING, CHRISTENSEN, ERLING
Applicants: CHRISTENSEN ERLING, CHRISTENSEN, ERLING
Application year: 1994
Main classification: B60C11
IPC codes: B60C11